A cat that chirps

I have noticed this strange phenomena with my own cats! Though I am not exactly sure, I will offer my best. There are two types of chirping my cats make. One is in the morning, when I first wake up. He will follow me around making strange quacking sounds or chirps, unlike his usual whiny meow later on. I'm not sure what causes this. The second, which is what I believe you are referring to, is when my cats see a bird/lizard/etc. and make a chirping sound before pouncing on it. So, it's not something particularly unusual; I believe most cats do this.